1. Field of the Invention
The present invention relates to an apparatus for welding joint sections of straps and others of a grid support, which constitute a part of a nuclear fuel assembly for an atomic reactor.
2. Description of the Related Art
Conventionally, a nuclear fuel assembly used in light water atomic reactors, for example, is comprised by a plurality of grid supports disposed between an upper nozzle and a lower nozzle that are separated at a given distance, and includes an instrument pipe and a plurality of control rod guide pipe fixed to each grid support, upper nozzle and lower nozzle, and fuel rods inserted in the grid spaces of each grid support. Each grid support is constructed by cross joining the straps made of thin strips in a lattice form.
For example, in the grid support 1 shown in FIG. 10, individual inner straps 2 are assembled to form a lattice structure containing a plurality of grid spaces 3, and the joint sections (weld sections) P formed by crossed straps 2 are spot welded using a laser welding apparatus and the like. Further, a sleeve 4 is inserted in a portion of the grid space 3, and the abutting sections Q formed at the cutout section 5 formed at the top and bottom ends of the inner strap 2 are seam welded in a continuous line by using a laser welder. A control rod guide pipe 6 is inserted in each sleeve 4, and the two are locked as a unit by expanding the tubes.
Also, at the intersection points of the inner straps 2 and the outer straps 8 shown in FIG. 11, weld tabs 7 formed on both ends of each inner strap 2 are coupled to respective grooves 8a of the outer straps 8, which are disposed on the four corners of the grid support 1, to form a coupling section R (welding section), which is also welded.
The grid support 1 thus assembled has an upper surface 1a and a lower surface 1b of a square shape, having many sections to be welded, such as a plurality of joint sections P and abutting sections Q, and rectangular shaped side surfaces 1c having the coupling sections R. Therefore, there is a need to carry out a large number of welding operations in making a fuel assembly.
When welding the grid support 1 with a laser welding apparatus and the like, fumes are produced so that welding operations are carried out in a hermetically sealed welding chamber 11 of a grid support welding apparatus 10, such as the one shown in FIG. 12, and it is necessary to replace the atmosphere inside the chamber 11 with an inert gas.
In the grid support welding apparatus 10, welding operations on the joint sections P and others on the grid support 1 placed inside the chamber 11 are carried out by using a laser welding device 12 disposed above the grid support 1. Specifically, a grid driving device (not shown) is used to move the grid support 1 inside the chamber 11 to weld successive sections, such as the joint sections P, abutting sections Q and coupling sections R. Also, an inert gas such as Ar and He is supplied to the chamber 11 from the lower section of the grid support 1 in a uniform dispersion by flowing through a feed pipe 13 and a diffuser plate 14 to pass through the grid support 1, and the spent gas is discharged from a discharge port 15.
However, when using the laser welding apparatus 12 to weld the welding sections of the grid support 1, a large quantity of fumes generated during the welding operations and the residual fumes remaining inside the chamber 11 adhere to the glass surfaces, resulting in a problem of fogging the glass surface of the chamber and lowering the transmission efficiency of laser power through the glass. Also, metallic vapors arising from the welding sections during the welding operations result in forming plasma gases, which can interfere with transmission of laser power or strike the glass surface to cause thermal damage to the glass surface.
However, in the grid support welding apparatus 10, although an inert gas is supplied to the interior space of the chamber 11, the inert gas only produces a flow path directing the inert gas from the lower surface 1b to the upper surface 1a of the grid support 1, and it is insufficiently for preventing the fumes from attaching to the glass surfaces or preventing the plasma from causing thermal damage to the glass surfaces. For these reasons, energy utilization efficiency of the laser device was inadequate, and thermal damage on the glass surfaces could not be prevented adequately.
It is an object of the present invention to provide a grid support welding apparatus to enable to effectively eliminate flying particles produced during the welding operation of a grid support.
The object has been achieved in a grid support welding apparatus comprising: a welding chamber for positioning a grid support for welding; a welding device disposed externally to the welding chamber for welding the grid support contained in the welding chamber; and a slit opening provided inside the welding chamber for jetting an inert gas stream along chamber surfaces of the welding chamber.
When performing welding operations of the grid support housed inside the chamber, fumes and plasmas are produced from the welded sections but they are swept away by the blast of inert gas streaming along the chamber surfaces and are prevented from adhering to the chamber surfaces and eliminated from the chamber. The chamber surfaces are thus kept clean and welding efficiency is maintained.
Also, the chamber surfaces may be positioned opposite to the welding sections of the grid support, because the fumes and plasmas can be effectively swept away by the blast of inert gas streams from the chamber surfaces.
Also, the blast sections may be provided near the intersections of a plurality of chamber surfaces, and inert gas streams may be ejected along the plurality of chamber surfaces in a plurality of directions. This design is highly efficient in preventing the fumes and plasmas from adhering to the chamber surfaces, because one blast section can produce streams in a plurality of directions.
In particular, when the chamber is made in a hexagonal shape, a pair of mutually opposing blast sections may be disposed on top and bottom orthogonal edge sections of two surfaces of the welding chamber respectively opposing an upper grid support surface and a lower grid support surface, each having aligned joint sections. This arrangement allows inert gas streams to flow along the six surfaces, and, especially, allows two inert gas streams to be ejected along the two surfaces opposing the upper and lower surfaces of the grid support that have many welding sections. Therefore, the efficiency of eliminating the fumes and plasmas are further improved.
Also, the blast section may blow out a planar gas stream along a surface of said welding chamber. The planar shape of the stream promotes reliable elimination of the fumes and plasmas.
In these cases, the blast section may be made of an integral section or a plurality of disconnected sections.